How to make particle instances the same size as the dupli object?
I completely forgot that there are actually two separate values which affect the size of the particles.  There is the hair length (found at the top of the settings), but there is also a size spinner under the physics rollout.



It turns out that the actual size of the particles is the product of these two values.  I.e. actual_size = length * size.  So if both these values are set to 1 the particles will be the same size as the dupli object.

The size defaults to 0.05, while length defaults to 4, so the actual size of the particles by default is 0.05 * 4 = 0.2, which is one fifth the dupli object's size.

I have no idea why there are two separate values which control particle size, or why one of them is under the physics rollout, or why they have such strange default values.  If someone could explain this I would greatly appreciate it!